High-voltage metal-oxide-semiconductor (HVMOS) devices are widely used in many electrical devices, such as input/output (I/O) circuits, CPU power supplies, power management systems, AC/DC converters, etc. There are a variety of forms of HVMOS devices. A symmetric HVMOS device may have a symmetric structure on the source side and drain side. High voltages can be applied on both drain and source sides. An asymmetric HVMOS device may have asymmetric structures on the source side and drain side. For example, only one of the source side and drain side, typically the drain side, is designed for sustaining high voltages.
FIG. 1 illustrates a conventional HVMOS device, which is also referred to as a double diffusion drain (DDD) MOS device. The HVMOS device includes gate oxide 102a, gate electrode 102b on gate oxide 102a, DDD 103 in substrate 101, and high-voltage junction 107 in DDD 103. Substrate 101 is of an opposite conductivity type than DDD 103. DDD 103 is lightly doped, and has a same conductivity type as high-voltage junction 107.
The conventional HVMOS device suffers from drawbacks. The breakdown voltage of the HVMOS device as shown in FIG. 1 is related to the distance S between high-voltage junction 107 and gate electrode 102b, and the greater distance S is, the higher the breakdown voltage will be. Therefore, to increase the breakdown voltage, the distance S has to be increased. However, the increase in distance S requires the HVMOS device to occupy a greater chip area.
An additional problem is that the breakdown voltage of the HVMOS device as shown in FIG. 1 is related to the distribution of the electrical field, particularly the electrical field between gate electrode 102b and high-voltage junction 107. However, it is difficult to adjust the distribution of the electrical field in the conventional HVMOS device. A solution for the above-discussed problems is needed.